PGP (Pretty Good Privacy) is a computer security program that provides cryptographic privacy and authentication. It is often used to encrypt and decrypt Email communications. PGP conforms to the OpenPGP standard for encryption and decryption. Early versions were full working freeware and will be well up to the task you require

http://www.oldversion.com/program.php?n=pgp

http://www.pgpi.org/products/pgp/versions/freeware/

The latest versions are only available as evaluation or full retail versions